1. Field of the Invention
This invention relates to a magnetic recording medium such as a magnetic tape, a magnetic card or a magnetic disk. More particularly, it relates to an improvement of a binder contained in a magnetic layer formed on a non-magnetic support.
2. Description of the Related Art
In general, a magnetic recording medium such as a magnetic tape or a floppy disk is prepared by coating a magnetic paint formed by dispersing a ferromagnetic powder together with an organic polymer as a binder in an organic solvent, on a polymeric film or sheet.
Due to a recent increase in the demand for a higher recording density and a higher S/N ratio of the magnetic recording material, the particle size of the magnetic powder has been made finer and the coercive force of the magnetic powder increased, and therefore, the dispersing capacity of the binder is an important factor when dispersing the magnetic powder uniformly in the magnetic paint, forming a smooth magnetic layer having a high degree of filling, and improving the performance of the binder.
As the dispersibility of the finely divided magnetic powder is increased, the viscosity of the magnetic paint is increased and problems often arise at the coating step. Therefore, it is necessary to increase the dispersibility of the magnetic powder while controlling the viscosity of the paint.
To improve the dispersion state of the magnetic powder and the viscosity characteristics of the magnetic paint, usually a method is adopted in which a surface active agent having a low molecular weight is used as the dispersant, but in view of the durability of the magnetic recording medium and a prevention of contamination of the head, the amount of dispersant used is naturally limited, and to improve the reliability of the magnetic recording medium, the binder must have a dispersing capacity. Separately, a method in which a curable compound represented by a polyisocyanate compound is incorporated into a paint, whereby the formed magnetic layer is crosslinked, is customarily adopted for increasing the durability and reliability of the recording medium, especially in the field of a recording magnetic tape, and in this case, the binder must have an appropriate reactivity with the polyisocyanate. If this reactivity is too high, however, the pot life of the paint is short and the paint is disadvantageous from the economical viewpoint. If the reactivity is too low, the desired improvement of the durability or travelling property cannot be obtained.
With a view to improving the binder, to cope with this increase of the performance of the magnetic recording medium, especially in connection with polyvinyl chloride (PVC) binders, the inventors investigated known PVC binders, and found the following. In a vinyl chloride/vinyl alcohol/vinyl acetate copolymer, the dispersing capacity for the magnetic powder is increased with an increase of the ratio of the vinyl alcohol, but the degree of improvement is low and the aid of a dispersant is necessary. Furthermore, as the proportion of the vinyl alcohol is increased, the pot life becomes short and the moisture resistance of the coating becomes low, and furthermore, the rate of thermal decomposition is increased. A vinyl chloride/maleic acid/vinyl acetate copolymer is fully adsorbed in the magnetic powder and exerts an excellent dispersing capacity, but an increase of the viscosity or gelation readily occurs and the reaction with the polyisocyanate tends to occur locally.
As the means for solving the problems of typical PVC binders, there have been proposed a method in which parts of OH groups of a vinyl chloride/vinyl alcohol/vinyl acetate copolymer are substituted with an organic group having a hydrophilic group such as COOM, S.sub.3 M, S.sub.4 M or PO(OM).sub.2 in which M represents hydrogen, an alkali metal or ammonium (Japanese Unexamined Patent Publication No. 57-44227), a method in which a vinyl chloride/vinyl acetate copolymer having a hydrophilic group as mentioned above is saponified (Japanese Unexamined Patent Publication No. 58-114330, No. 61-57640 and No. 61-96515), and a method in which a vinyl chloride/vinyl alcohol/vinyl acetate copolymer having an anionic hydrophilic group introduced therein us used. According to these proposals, the dispersibility is improved but the level of improvement level is still too low, and since these methods include the saponification step, the heat stability of the polymers is poor and in magnetic recording media formed by using these polymers, there is a risk of a reduction of the reliability thereof with time.
Furthermore, a method has been proposed in which a copolymer of a hydroxyl group-containing monomer such as hydroxylethyl (meth)acrylate or hydroxylpropyl (meth)acrylate with an ion-dissociative hydrophilic group-containing monomer is used so that deterioration of the polymer by the saponification is avoided (Japanese Unexamined Patent Publication No. 56-77930, No. 60-185226 and No. 60-235814). In this case, the dispersibility and heat stability are improved, compared with the case where the polymer comprising vinyl alcohol units is used as the hydroxyl group-containing units, but the improvement is still unsatisfactory. Furthermore, in this method, as the dispersibility is increased, the viscosity of the paint is increased, and problems readily arise at the coating step.
Furthermore, a method has been proposed in which the dispersibility of a magnetic powder is improved by introducing a cationic hydrophilic group represented by a quaternary ammonium salt in polyvinyl chloride containing a hydroxyl group (Japanese Unexamined Patent Publication No. 62-73416 and Japanese Unexamined Patent Publication No. 62-134819). Nevertheless, the pot life after the addition of an isocyanate compound is short and problems readily arise with time in the coating process, although the viscosity of the magnetic paint before the addition of the isocyanate compound is low.
A lubricant represented by a fatty acid is added to a magnetic paint to maintain a good travelling performance, but if the heretofore proposed vinyl chloride binders contain a hydroxyl group in an amount sufficient to impart a required degree of crosslinking by the reaction with the polyisocyanate, as pointed out above, the problem arises of a reduction of the dispersion stability of the magnetic paint, and this tendency becomes conspicuous as the particle size of the magnetic powder is decreased.